The following is not an admission that anything discussed below is common general knowledge or citable as prior art.
Immersed MF or UF membranes may be made in a flat sheet, alternatively called plate and frame, configuration. In this configuration, a roll of membrane sheet is made by casting a polymeric separation layer cast onto a roll of non-woven substrate. Two generally rectangular pieces of membrane sheet are attached at their edges, for example by supersonic welding, onto opposing sides of hollow plastic frame. This creates a panel with a hollow interior channel to collect filtered water, alternatively called permeate. Several panels slide side by side into a frame that can be immersed in water to be filtered. The insides of the panels are connected to the suction side of a pump to draw permeate through the membrane sheets. Bubbles provided from below the frame cause a mixed flow of bubbles and liquid to rise through vertical slots between the panels to held clean the membrane surfaces. Examples of this type of device are shown in U.S. Pat. Nos. 5,482,625; 5,651,888; 5,772,831; 6,287,467; and, 6,843,908, all owned by Kubota Corporation.
Flat sheet membrane modules are generally robust and have a low manufacturing cost (relative to hollow fiber membranes) because they can be cast in a wide sheet. However, flat sheet membranes have poor packing densities (membrane surface area per unit volume of the module) relative to hollow fiber membranes and so the total cost of a large flat sheet plant can be very high. Further, typical flat sheet membrane panels cannot be backwashed vigorously enough to clean the membranes mechanically.
A variation of a flat sheet membrane element is shown in International Publication Number WO 2007/036332 to Microdyn-Nadir GMBH. In these elements, two layers of membrane material are cast onto the front and back sides of a fabric having a porous central area between two dense layers. The central area provides a permeate channel also connects the two dense layers together allowing the element to be backwashed for mechanical cleaning. These elements do not require a four-sided frame and they are about 2 mm thick, which is thinner than the plate and frame elements described above. However, these elements are also flexible and they are spaced apart by about 10 mm center to center in a frame. The packing density is better than for the plate and frame elements described above, but still much lower than a hollow fiber membrane module. The central area of the fabric, although porous, also provides a resistance against flow in the permeate channel and increases the cost of the element.